1. Field of the Invention
This invention relates to a focus detecting apparatus of high accuracy best suited for use as the automatic focusing apparatus of an optical microscope, a camera or the like.
2. Description of the Prior Art
An automatic focus detecting apparatus as disclosed, for example, in U.S. Pat. No. 4,311,904 is known. In such apparatus, the image signal of an object image by an image sensor has been obtained, a signal which is a high frequency component of a magnitude corresponding to the contrast such as light and dark (hereinafter referred to as the edge signal) has been obtained from the image signal and the edge signal has been integrated to obtain a signal, corresponding to the deviation from the in-focus point. The edge signal corresponds to the boundary of the contrast of the object image formed by the image sensor.
In such apparatus, as the object image on the light-receiving surface of the image sensor becomes approximate to the in-focus state (as the deviation from the in-focus point becomes approximate to zero), the peak value of the aforementioned edge signal becomes a signal which is gradually greater in magnitude and gradually narrower in width. As a general tendency, the output signal of an integration circuit which integrates the edge signal rises as the in-focus state is approached, and assumes a maximum value during the in-focus condition, but in some cases, it exhibits some decrease depending on the object, and this has led to the possibility of degrading the focusing accuracy.
Also, where the contrast on the light-receiving surface is so small that for example, the number of boundaries between the light and the dark is one, the amount of integration by the integrating circuit is small, and this has led to the problem that S/N is low and an out-of-focus signal is not obtained.